As one technique for bonding two metal materials, there has been known an electroforming process (metal product production or reproduction technique based on an electroplating process). However, the electroforming process requires a fairly long time for forming an electroformed layer (plated layer), which leads to an increase in cost. Therefore, as a technique for bonding two metal materials, a pulse electric-current bonding process (also called “SPS (Spark Plasma Sintering) process”) as disclosed in the following Patent Document 1 is often employed. The pulse electric-current bonding process is designed to bond two metal materials together by causing an electric current to flow through the metal materials so as to heat them, while applying a load thereto. Thus, the bonding between the metal materials can be achieved in a short period of time and at a low cost, as compared with the electroforming process.